1. Field of the Invention
The present invention relates generally to the field of electronics. More particularly, the present invention relates to a clock circuit that selectively drives a clock signal to an option module via a transmission line when the option module is coupled to the clock circuit, and prevents propagation of the clock signal through the transmission line when the option module is decoupled from the clock circuit.
2. Description of Art Related to the Invention
It is well known that many electronic systems (e.g., computers, workstations, mainframes, etc.) are designed with a number of printed circuit boards which are electrically coupled together. One of these printed circuit boards, referred to as the "motherboard", includes a clock generation circuit (e.g., crystal oscillator) that produces a "master clock signal", being the main clock utilized by the electronic system. The master clock signal is replicated to generate a plurality of copies, which are referred to as "system clock signals". Each of the system clock signals is driven to different portions of the electronic system by a clock driver. For those outdated system architectures utilizing low-speed clock frequencies (e.g., 33 megahertz "MHz" or less), the clock drivers typically employ Transistor-Transistor logic ("TTL") or Complementary Metal-Oxide Silicon ("CMOS") logic.
Of the plurality of system clock signals produced, a number of these system clock signals may be routed by transmission lines to removable option modules such as graphics cards, processor cards and the like. Each transmission line may include, but are not limited to a first segment of a printed trace line implemented on the motherboard, a second segment of a printed trace line implemented on an option module. Of course, the connectors coupling the first and second segments of printed trace lines influence the impedance of the transmission line. When coupled to the motherboard, an option module provides enhanced functionality to the electronic system.
In order to preserve signal integrity of each system clock signal and mitigate the effects caused by electromagnetic interference "EMI", each transmission line must be terminated. Such termination may be accomplished, for example, by placing a termination resistor proximate to a load receiving a system clock signal. The termination resistor is configured with impedance equal to that of the transmission line and is coupled to both ground and the transmission line.
Since it is desirous for the termination resistor to be in close proximity to the load, the termination resistor usually is placed on the option module. Thus, when the option module is implemented within the electronic system by connecting the option module to one of the connectors, the transmission line associated with the connector is properly terminated. However, when the option module is removed from the electronic system, the transmission line associated with that option module has no termination. Hence, the transmission line radiates EMI along the transmission line creating difficulties in meeting Federal guidelines on EMI limits produced by computers and other electronic systems.
For many years, this problem has been overcome by connecting a pair of reversed biased diodes (e.g., Shottkey diode clamp) to the transmission line in such a fashion to maintain the voltage of the system clock signal between two voltage parameters. For example, anode of a first diode may be coupled to a supply voltage reference (e.g., a +5.0 volt "V" supply) and a cathode of a second diode may be coupled to a ground reference. As a result, the first and second diodes maintain the voltage parameters ranging from approximately +5.4 V and approximately -0.4 V for a CMOS clock driver, taking into account forward conduction voltage of the diode being equal to approximately 0.4 V.
With advancements in the electronic systems resulting in the use of high clock frequencies (e.g., over 100 MHz), the clock drivers are being implemented with either Emitter Coupled Logic ("ECL") or Positive Emitter Coupled Logic ("PECL"). Thus, the conventional architecture of the clocking circuitry now has become subject to a number of disadvantages. One primary disadvantage is that the conventional clocking circuitry fails to eliminate propagation of a clock signal along the transmission lines when the option module is removed from the electronic system. As a result, as the clocking frequency used by the electronic system increases, the EMI radiating from the transmission lines increases. Therefore, it is more difficult to design faster electronic systems employing conventional clocking circuitry due to an increase in skew between the system clock signals. Likewise, it is more difficult to meet EMI limits imposed by Federal regulations.
Hence, it would be advantageous to design a clock circuit which would provide a low-skew clock signal to the option module via one or more transmission lines when the option module is electrically coupled to the motherboard, and alternatively, discontinue supplying the low-skew clock signal to the connector reserved for the option module after the option card is removed therefrom by deactivating the transmission line(s).